1. Field of the Invention
This invention relates to an image forming apparatus such as a copying apparatus, a printer, a recorded image displaying apparatus or a facsimile apparatus for developing an electrostatic latent image formed on an image bearing member by an electrophotographic system or an electrostatic recording system or the like and forming a visible image, and to a developing device of the image forming apparatus.
2. Related Background Art
There is known a developing device in which a dry type developer as a visualizing agent is carried on a surface of a developer bearing member and this developer is conveyed and supplied to the vicinity of the surface of an image bearing member bearing an electrostatic latent image thereon, and the electrostatic latent image is developed into a visible image while an alternating electric field is applied to between the image bearing member and the developer bearing member.
A developing sleeve is generally often used as the developer bearing member and therefore, the developer bearing member will hereinafter be referred to as the "developing sleeve", and a photosensitive drum is generally often used as the image bearing member and therefore, the image bearing member will hereinafter be referred to as the "photosensitive drum".
As the developing method, there is known the so-called magnetic brush developing method comprising forming a magnetic brush on the surface of the developing sleeve having a magnet disposed therein by a developer (two-component developer) composed, for example, of two-component based composition (carrier particles and toner particles), causing this magnetic brush to rub with or be proximate to the photosensitive drum opposed to the developing sleeve with a minute developing gap held therebetween, and continuously applying an alternating electric field to between the developing sleeve and the photosensitive drum (between S-D) to thereby repetitively effect the transference of the toner particles from the developing sleeve side to the photosensitive drum side and the counter-transference to effect development. (See, for example, Japanese Patent Application Laid-Open No. 55-32060 and Japanese Patent Application Laid-Open No. 59-165082).
A developing device for the above-described two-component magnetic brush development is provided with a developing container comparted into a developing chamber and an agitating chamber by a partition wall, and agitating and conveying screws which are agitating members are rotatably contained in the developing chamber and the agitating chamber. In the opening portion of the developing chamber, a developing sleeve rotated in a predetermined direction is disposed in opposed relationship with a photosensitive drum rotated in a predetermined direction, with a minute spacing therebetween, and a magnet is fixedly disposed in the developing sleeve.
A developer comprising a mixture of toner particles and magnetic carriers is contained in the developing container, and the mixture ratio (hereinafter referred to as the "T/C ratio") of the toner particles and the magnetic carriers is kept constant by an amount of toner corresponding to the toner consumed by development being dropped and supplied from a toner storing chamber in which a toner for replenishment is contained.
The dropped and supplied toner is agitated with the developer in the developing container by the screw in the agitating chamber and conveyed. The supplied toner is conveyed along the lengthwise direction of the container conversely to the direction of conveyance of the developer by the conveying screw in the developing chamber. Openings are formed in this side and the inner side of the partition wall, and the delivery of the developer is effected in this opening portion.
Now, the maintenance of the mixture ratio of the toner particles and magnetic carriers of the two-component developer in the developing container is very important for the stabilization of an output image, and various types of methods of detecting and maintaining it have heretofore been proposed. There have been proposed and put into practical use, for example, a method of a type in which detecting means is provided around a photosensitive drum and light is applied to a developed toner image on the photosensitive drum and from the transmitted light or the reflected light at this time, the amount of toner supply is adjusted and as the result, the T/C ratio is detected, a method of a type in which detecting means is provided near a developing sleeve and the T/C ratio is detected from the reflected light when light is applied to a developer applied onto the developing sleeve, and a method of a type in which a sensor is provided in a developing container and by the utilization of the inductance of a coil, an apparent change in the magnetic permeability of the developer in a predetermined volume near the sensor is detected to thereby detect the T/C ratio.
However, the method of the type in which the T/C ratio is maintained from the amount of developing toner on the photosensitive drum suffers from the problem that for example, by the fluctuation of the gap between the photosensitive drum and the developing sleeve, the potential of a latent image or the like, the amount of developing toner fluctuates independently of the T/C ratio of the developer in the developing container and as the result, the proper supply of the toner becomes impossible, and the method of the type in which the T/C ratio is detected from the reflected light when light is applied to the developer applied onto the developing sleeve suffers from the problem that an accurate T/C ratio cannot be detected when the surface of the reflected light detecting means is stained by the scattering of the toner occurring when the charging amount of the toner is reduced under high humidity environment or the like.
In contrast with these, the method of the type in which by the utilization of the inductance of the coil, the variation in the magnetic permeability of the developer in a predetermined volume near the sensor is detected to thereby detect the T/C ratio (hereinafter referred to as the "inductance detecting sensor") is low in the cost of the sensor and in addition, is scarce in the wrong detection as described above and can accurately detect the T/C ratio of the developer.
The inductance detecting sensor is disposed near a screw, and on the basis of such a sequence that when for example, the magnetic permeability of the developer in a predetermined volume becomes great, it judges that the T/C ratio of the developer has become low, and starts the supply of the toner, and when conversely the magnetic permeability becomes small, it judges that the T/C ratio of the developer has become high, and stops the supply of the toner, it controls the T/C ratio of the developer.
In recent years, in image forming apparatuses, and particularly full color copying apparatuses, the downsizing of the apparatus has been required and along therewith, developing devices are in a situation wherein they must pursue further downsizing. As the result, they must use not only developing containers, but also developing sleeves and agitating members which are downsized, and form apparatuses of as high reliability as before.
On the other hand, the above-described inductance detecting sensor detects any change in the magnetic permeability of the developer in a predetermined volume and therefore, there arises the problem that when there is a fluctuation of the bulk density of the developer by being left as it is or the fluctuation or the like of the environment, it judges that the magnetic permeability differs in spite of the same T/C ratio and therefore, in order to cope with such problem, this sensor is usually disposed near the agitating member by which the developer is stably circulated and flows.
At this time, the following problem may arise depending on the relation among the agitating member of a small diameter and the bulk height of the developer, and the shape and size of the sensor.
When as shown in FIG. 10 of the accompanying drawings, the size of the detecting surface of a sensor 110, e.g. the diameter thereof when the detecting surface is substantially circular, is considerably large relative to the rotation diameter of an agitating member 105 and a developing container substantially along the curvature thereof, there are formed spaces as indicated by hatched portions c and d in the gap between the sensor 110 and the agitating member 105.
When in the presence of such spaces, the developer is agitated in the developing container 101, the developer which has come into the spaces indicated by the hatched portions c and d, particularly in the portion c, is not conveyed by the agitating member 105 but stagnates.
It is chiefly the developer in a hatched portion e circulated in the developing container 101 by the agitating member 105 that the T/C ratio of the developer varies for the consumption and supply of the toner by the developing operation and therefore, the developer present in the spaces indicated by the hatched portions c and d wherein the developer stagnates, particularly in the portion c, is very small in the fluctuation of the T/C ratio.
If in this state, an attempt is made to detect the T/C ratio by the inductance detecting sensor 110, the stagnant developer in the hatched portions c and d wherein the change in the T/C ratio of the developer is small is also detected with the developer in the hatched portion e wherein the T/C ratio fluctuates and therefore, there cannot be obtained the output value of the toner density detecting sensor 110 which accurately corresponds to the T/C ratio.
In FIG. 11 of the accompanying drawings, a straight line X shows the relation of the output value of the toner density detecting sensor to the T/C ratio of the developer. The straight line X is an ideal line.
The relation of this straight line X is an ideal state, and even when the consumption and supply of the toner are effected from the center T/C ratio, an error will occur to the amount of toner supply unless the T/C ratio shifts on the straight line X. In contrast, a straight line Y in FIG. 11 shows a variation in the output value of the inductance detecting sensor when the above-mentioned spaces are present and the consumption and supply of the toner are actually effected. From the straight line Y, it will be seen that when the T/C ratio becomes low, the output of the inductance detecting sensor tends to become low as compared with the case of the straight line X, and when the T/C ratio becomes high, the output of the inductance detecting sensor tends to become high as compared with the case of the straight line X.
This is because even if the T/C ratio of the developer in the hatched portion e circulated in the developing container is reduced, the stagnant developers in the hatched portions c and d remains approximate to the center T/C ratio and as the result, the inductance detecting sensor detects both of the developers low in the T/C ratio and therefore, the output value becomes low relative to the output value for the straight line X and even if conversely the T/C ratio of the developer in the hatched portion e rises, the stagnant developers in the hatched portions c and d remain approximate to the center T/C ratio and therefore, the inductance detecting sensor detects both of the developer high in the T/C ratio and the developer of the center T/C ratio and therefore, the output value becomes high relative to the output value for the straight line X.
For the reason set forth above, there cannot be obtained the output value of the inductance detecting sensor which accurately corresponds to the T/C ratio, and if in this case, the stagnant developers in the hatched portions c and d do not move, the sensor sensitivity (the amount of change in the output of the sensor for a change of 1% in the T/C ratio) drops, but if the output value of the inductance detecting sensor for a change in the T/C ratio changes always on the straight line Y, the change in the T/C ratio can be sufficiently detected.
However, when the stagnant developers move due to the vibration of the copying apparatus itself, the vibration of the developing device by the copying operation, a change in the fluidity of the developer, a change in the bulk density of the developer, etc., the T/C ratio following line Y is not reproduced.
When conversely, the size of the detecting surface of the sensor, e.g., the diameter thereof when the detecting surface is substantially circular, is considerably small relative to the rotation diameter of the agitating member and the container substantially along the curvature thereof, the above described problem of dead space is solved, but first, there arises the problem of a reduction in the absolute output of the sensor. This reduction in the absolute output can be prevented by improving members such as a coil and a core in the sensor, but in that case, an increase in cost results. Also, if the detecting area of the sensor becomes small, the possibility of detecting a local change in the magnetic permeability of the developer in the developing container (for example, the developer locally including the coagulated toner) becomes high and as the result, again in this case, a wrong toner supplying operation will occur.
Also, the wrong detection by the inductance detecting sensor may also occur from the relation between the bulk height of the developer present in the portion wherein the agitating member is disposed and the location at which the sensor is disposed. This is liable to occur particularly when the sensor is disposed on the side wall surface of the container near the agitating member. Usually, the bulk height of the developer (the surface of the developer) in the portion in an agitating chamber R2 wherein the agitating member is disposed is such that in order to satisfy good agitation, as shown in FIG. 12 of the accompanying drawings, about 75% to 90% of the outermost rotational surface of the agitating member is buried. If at this time, the sensor disposed on the wall surface on the side of the agitating member is too much above the rotational center axis of the agitating member, the uppermost surface of the sensor will be located above the uppermost surface of the developer and thus, there will occur the phenomenon that the detection output decreases sharply.
On the other hand, the developer present in the gap between the lower portion of the agitation member indicated by a hatched portion h in FIG. 12 and the inner wall surface near the bottom of the container is somewhat low in flow speed as compared with that in the upper portion, and is liable to stagnate particularly under high humidity environment. Again when the detecting surface of the sensor hangs over this portion, the accuracy of the output is reduced.
Consequently, it is desired to make the positional relation and the size relation between and the shapes of the small-diametered agitating member and the inductance detecting sensor and the gap therebetween proper.